Final answer:
There are seven families of DNA-containing viruses that infect vertebrates, categorized based on their distinctive characteristics and mechanisms of infection.
Step-by-step explanation:
The question "There are _______ families of DNA-containing viruses that infect vertebrates" is asking about the number of recognized families of DNA viruses that have vertebrates as their hosts. The correct answer is D) seven. In virology, DNA viruses that infect vertebrates are categorized into seven different families, each with its own distinctive characteristics and mechanisms of infection. These families include, for example, the Herpesviridae, known for causing herpes infections, and the Adenoviridae, known for causing respiratory illnesses among other diseases.
Chapters 6 and 21.3 in the given reference material likely provided the insights into the classification and characteristics of viral families. For instance, Group VI viruses are known for having ssRNA genomes and using reverse transcriptase to integrate into the host genome. Group VII viruses, on the other hand, have partial dsDNA genomes and also use reverse transcriptase for replication. These details about viral genetics help in the classification and understanding of different viral families.